Wireless communications is a rapidly growing segment of the communications industry, with the potential to provide high-speed high-quality information exchange between portable devices located anywhere in the world. Potential applications enabled by this technology include multimedia internet-enabled cell phones, smart homes, appliances, automated highway systems, distance learning, and autonomous sensor networks, just to name a few. Supporting these applications using wireless techniques poses significant technical challenge. As handsets move to meet broadband, the requirements of components are more astringent. Electrical communication systems demand new more efficient low loss devices that can be used at higher frequency ranges.
Recent advances in tunable ferroelectric materials have allowed for relatively low capacitance varactors that can operate at temperatures above those necessary for superconduction and at bias voltages less than those required for existing planar varactor structures, while maintaining high tenability and high Q factors. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called “ferroelectric” because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate BaxSr1-x TiO3 (BST) or BST composites have been the subject of several patents. Dielectric materials including BST are disclosed by Sengupta, et al. in U.S. Pat. No. 5,312,790; U.S. Pat. No. 5,427,988; U.S. Pat. No. 5,486,491; U.S. Pat. No. 5,846,893; U.S. Pat. No. 5,635,434; U.S. Pat. No. 5,830,591; U.S. Pat. No. 5,766,697; U.S. Pat. No. 5,693,429; U.S. Pat. No. 6,074,971; U.S. Pat. No. 6,801,104 B2 and U.S. Pat. No. 5,635,433. These patents are hereby incorporated by reference. The permittivity (more commonly called dielectric constant) of these materials can be varied by varying the strength of an electric field to which the materials are subjected. These materials allow for thin-film ferroelectric composites of low overall dielectric constant that takes advantage of the high tunability and at the same time having high dielectric constants.
BST thin films have been used in microwave circuit applications because of their high dielectric constant, high tunability, low loss, and fast switching speed. Tunable BST films have been demonstrated as an attractive technology to low cost agile mobile circuits, such as tunable filters, tunable matching networks at a high tunable frequency range. Most of these technologies have focused on material quality, choice of electrodes, and deposition or processing techniques. As the technology matures, several issues have arisen due to the deposition methods and limited choice of electrode materials available due to the extreme deposition temperatures. Quality factor, resonance frequency and breakdown voltage are important factors for determining which applications BST thin films will work best in. High-frequency device losses consist of material-related losses in the film and at the electrode-film interface, as well as the resistive losses in the electrodes. First, thermal strain on the interface between the ferroelectric thinfilm and the metal electrode due to the creation of oxide films and crystalline microstructure. This interface is generally the cause of losses at high frequencies and premature breakdown at low voltages. Second, the designs on current devices are limited by traditional design guidelines that create resistive losses due to design constraints.
There is a need in the industry to improve the efficiency of BST thinfilm capacitors by design implementation. There is a further need to create a BST thinfilm design structure that minimizes loss at the dielectric-electrode interface. There is also a further need to create BST thinfilm designs that presents and improved structure where there is minimum contact with the bottom electrodes and creates an optimized periphery with a superb quality (Q) factor and a reasonable aspect ratio range.